The present invention is generally related to electronic circuits, and, more particularly, to method and processor for reducing the level of feedback-induced noise in an automatic gain control circuit for audio level control in a radio transmitter.
Users of Specialized Mobile Radio (SMR) systems, such as Land Mobile Radio (LMR) systems, etc., commonly used in dispatch applications, where a large number of users may share a single base station, are accustomed to fairly consistent recovered audio levels, especially when using analog frequency modulation (FM) communication techniques. The audio level consistency in this communication technique may be achieved by using an automatic gain control circuit (AGC) in the audio path or by using a combination of amplification and limiting of the FM deviation. This allows for different audio levels applied into the transmitting radio microphone to be received at a somewhat constant level on the receiving radios within the system. With the introduction of digital voice systems, it was noticed that such systems also suffer from the lack of consistency in recovered audio levels. Digital AGC circuits can be used in this case to recover a consistent audio level. However, as further elaborated below, the latency inherent in digital voice systems causes an additional complication. That is, the latency of digital voice systems results in these systems suffering from severe feedback-induced noise when receiving radios are near a transmitting radio. For example, the audio output from the speakers of the receiving radios can be fedback into the transmitting radio microphone causing unacceptable distortion and undesirable increase in gain from the AGC circuit.
When digital voice coders, i.e., digital vocoders, were developed for LMR applications one of the main goals was faithful reproduction of voice. However, differences in operation between analog and digital-based voice communication systems were noticed. Unlike an analog-based system, a digital-based voice system is substantially impervious to the presence of noise in the communications channel, except for bit errors that manifest themselves mainly as audio artifacts and not noise. One known characteristic of vocoders is that, for the most part, they are linear gain devices; essentially whatever level goes into the device, comes out. Thus, if the audio level is low at the transmitting radio microphone input, the audio level will be similarly low at the receiving radio speaker output. Further, vocoders are commonly used in trunked LMR systems where each conversation can consist of multiple transmissions from different users. The received audio level from each user can vary based on a myriad of factors, such as the user""s voice level, how they hold the microphone, etc. To compensate for these factors, presently available AGC circuits have proved to be somewhat effective. However, as suggested above, there is also inherent latency in digital speech caused by processing and transmission delays. This latency in some radio systems can be on the order of several hundred milliseconds. The latency aggravates feedback-induced noise when other receiving radios are near the transmitting radio.
Typically in LMR applications, the users do not hold the radio speaker close to their ear. For understandable reasons, users, such as police officers, fire fighting personnel, emergency first aid personnel, operators of vehicle fleets, public utilities personnel, etc, that need unimpeded use of their hands, simply carry their portable radios on a belt-attached holster or equivalent and set their radio sufficiently loud to be able to quickly monitor and respond to communications addressed to a given user or group of users. The speakers on mobile and portable radios can be acoustically loud, and the volume can be turned up high especially in a high background noise environment. In this case, the audio output from the speakers of any neighboring receiving radios can be fedback into the transmitting radio microphone causing severe distortion and undesirable increase in the gain of the AGC circuit and thus compromise the efficacy of such a circuit.
As suggested above, one known approach to ameliorate the lack of consistent audio level is the inclusion of the AGC circuit in the audio path after the microphone. Unfortunately, known AGC circuits generally constitute circuits with fixed gain profile and response time. Typically, these circuits cannot be easily modified to adapt to changing operational conditions. It is believed that prior to the present invention no solution has been proposed to effectively suppress the foregoing feedback-induced noise that has affected LMR systems.
Thus, in view of the foregoing issues it would be desirable to provide digital signal processing techniques that would allow for reducing the level of feedback-induced noise in the output signal from an audio automatic gain control circuit in a radio transmitter while providing a substantially constant audio to that signal.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof, a method for reducing the level of feedback-induced noise in the output signal from an audio automatic gain control circuit in a radio transmitter. The method allows for receiving a stream of pulses, e.g., PCM audio samples, comprising an input signal to the automatic gain control circuit. The method further allows for receiving estimates of respective high and low frequency energy components of the input signal. The respective high and low frequency components are averaged, e.g., over a respective sliding window. An energy scalar is calculated based on the ratio of a predefined target energy level over a combined value of the high and low frequency components. A relating action relates the target energy level to the combined value of the high and low frequency components. Based on the relating results, the calculated energy scalar is limited to within two limit values. Another relating action relates the values of the averages of the high and low frequency components to one another. If the value of the high frequency average exceeds the value of the low frequency average, the energy scalar is reduced to a value sufficiently low to suppress the presence of feedback-induced noise in the input signal of the circuit, and generate an output signal with a corresponding low level of feedback-induced noise. If the value of the low frequency average exceeds the value of the high frequency average, the energy scalar is applied to the input signal to generate an output signal scaled within the two limit values.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a processor for reducing the level of feedback-induced noise in the output signal from an audio automatic gain control circuit in a radio transmitter. The processor includes at least one port for receiving a stream of pulses comprising an input signal to the automatic gain control circuit, and for receiving respective estimates of high and low frequency energy components of the input signal. An averaging module is configured to average the respective high and low frequency components over a respective sliding window. A calculating module is configured to calculate an energy scalar based on the ratio of a predefined target energy level over a combined value of the high and low frequency components. A comparator is configured to relate the target energy level to the combined value of the high and low frequency components. A limiter is responsive to the comparator to limit the calculated energy scalar to a range between two limit values based on the relating results from the comparator. A comparator allows relating the values of the averages of the high and low frequency components to one another. A noise-reduction processing module is responsive to the comparator for relating the values of the averages of the high and low frequency components to one another to perform the following actions:
If the value of the high frequency average exceeds the value of the low frequency average, reducing the set energy scalar to a value sufficiently low to suppress the presence of feedback-induced noise in the input signal of the circuit, and generate an output signal with a corresponding low level of feedback-induced noise; and
If the value of the low frequency average exceeds the value of the high frequency average, applying the energy scalar from the limiter to the input signal to generate an output signal scaled within the two limit values.